The present invention relates to an improved sealed lead-acid battery of the gas recombination type wherein a liquid electrolyte is immobilized and oxygen gas evolved at the positive plate is caused to react with the negative plate.
In a conventional sealed lead-acid battery of this type, the liquid electrolyte is immobilized either in the form of a gelled fine silica powder or in the form of a mat typically made of glass fibers, and oxygen gas evolved at the positive plate moves to the negative plate and undergoes a "sealed" reaction therewith. In such a lead-acid battery, sulfuric acid in the upper part of the electrolyte is consumed in the discharging process whereas, in the charging process, sulfuric acid is released from the plates into the electrolyte and sinks to the bottom. Therefore, as discharge-charge cycles are repeated, the electrolyte stratifies, producing an upper layer containing a low concentration of the electrolyte and a lower layer containing a high concentration of the electrolyte.
In ordinary flooded lead-acid batteries using a free flowing electrolyte, stratification of the electrolyte can be readily recovered by agitation such as by gassing when in the overcharged condition or by pumping a gas into the electrolyte. On the other hand, in the sealed lead-acid battery of the type contemplated by the present invention wherein the electrolyte is immobilized, only a small degree of electrolyte stratification occurs during discharge and charge reactions, and such stratification proceeds very slowly. Additionally, due to the effects of gravity, more of the electrolyte is present at the lower part of the electrode plates or separator than at the upper part, with the result that the sealed reaction has a tendency to occur at the upper part of the negative plate more easily than at the lower part. As will be described below in more detail, however, water is formed as a result of the sealed reaction, and this makes it very difficult to avoid the occurrence of electrolyte stratification. Moreover, since the immobilized electrolyte in the sealed lead-acid battery is difficult to agitate, it is practically impossible to eliminate stratification of the electrolyte, resulting in an unavoidable capacity drop and sulfation in the lower part of the electrode plates.